How can I insert special iframe placeholder images?

Set a class (class="myvideoframe") to your video iframes. With it you can create your own fake elements.

Example:

var attributes = element.attributes;
if(attributes[ 'class' ] == 'myvideoframe')
{
    return editor.createFakeParserElement( element, 'cke_myvideoframe', 'iframefield', false );
}

For more infos how it works search the forum and take a look in source files.